Telephone privacy



Dec.. 5, 1961 R. R. RlEsz TELEPHONE PRIVACY 3 Sheets-Sheet 1 Filed NOV. 22, 1941 him O lll s" 129% .bfi

/A/ VEA/TOR R. R. R/E SZ A Tram/Ey 5 Sheets-Sheet 2 /Nf/E/v TOR R. R. R/E SZ A TroR/VEV n Mm. :n h R @EDI W c NK NM. mw

R. R. RIESZ TELEPHONE PRIVACY Dec. 5, 1961 Filed Nov. 22, 1941 Dec. 5, 1961 R. R. RlEsz TELEPHONE PRIVACY 5 Sheets-Sheet 5 Filed Nov. 22, 1941 /NVEA/TOR By R R/E SZ artes arent Office 3,012,098 TELEPHONE PRIVACY Robert R. Riesz, Chatham, NJ., assignorto Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York Filed Nov. 22, 1941, Ser. No. 420,052 11 Claims. (Cl. 179-15) The present invention relates to the secret transmission of speech or similar signals.

The invention is especially applicable to systems in which the speech or signal waves are analyzed into low frequency speech-defining signals before transmission. Systems of this type are known in the art and are disclosed, by Way of example, in Dudley Patent 2,151,091,

issued March 21, 1939, and Riesz Patent 2,183,248, issued December l2, 1939. Such systems of themselves possess a considerable degree of secrecy since it is irnpossible to reproduce understandablespeech from the transmitted currents without the use of specially constructed apparatus.

The present invention achieves a high degree of secrecy by operating on the low frequency waves that are transmitted in such a 'Way as to render them incapable of reproducing intelligible speech without knowledge of the type of transformation used at the transmitting point.

By changing the type of transformationffrom time to time, and especially inrandom manner, discovery of the key is rendered difficult and unlikely.

Referring to the drawings, FIG. l is a schematic circuit diagram in block form of a system incorporating the present invention; y

FIG. 2 is a detail showing of one of the modulator or demodulator circuits used in FIG. l;

FIGS. 3 and 4 show circuit characteristics to be referred to in the description; t

FIGS. 5 and 6 when placed together with FlG. 5 at the left show an alternative system in schematic diagram incorporating the invention; and Y FIG. 7 shows wave forms to be referred to in the description. v v

Referring first to FIG. l, there is shown at T apparatus for analyzing speech waves and controlling by the analyzed Awaves a series of carrier waves which are transmitted over the line L to the distant receiving point where they are received, detected and synthesized to speech by the receiving circuit R. rIhis system is substantially in accordance with that disclosed in the Riesz patent referred to except as modified in accordance with the present invention, as will presently be described.

The speech input Waves are picked up from a lineor,

as shown, in a microphone .1 and transmitted through v equalizer 2 to the speech analyzer. This consists of a pitch control channel 3 and a number of spectrum channels 4, assumed in the present disclosure to comprise ten such spectrum channels. v

The pitch control channel may be constructed in accordance with the Riesz patent disclosure and is shown as comprising amplifier 5, rectifier 6, frequency'measuring circuit 7, low-pass lter 8, modulator 9 and band-pass filter 10. As disclosed in the Riesz patent the output of the low-pass filter 8 is a slowly varying direct current the amplitude of which is a measure of the frequency of the fundamental or vocal cord sound. Thecharacter of this varying direct current may be seen from oscillograms published in FIG. 14 of a paper entitled Remaking Speech by H. Dudley, published in the Journal of the Acoustical Society of America, volume ll, pages 169 to 177, October 1939.

Each of the spectrum defining channels 4 includes a band-pass filter 11 followed by detector 12 and low-pass filter 13 leading in turn to a translating modulator 14 2 and band-pass filter 15. The pass-bands of the ten bandpass filters 11 'may have any convenient or suitable values but are indicated as having the same values as in the Riesz patent disclosure and the article referred to. The character of the output current from each low-pass filter 13 may be seen from the oscillogram shown in FIG. 14 of the article referred to. The equalizer 2 may be designed to have a loss varying with frequency in such manner as to equalize the output currents from the lowpass filters'13 so as to make the4 currents obtained by analysis of the higher speech frequency bands of cornparable amplitude to those obtained by analysis of the low frequency speech bands.v

The slowly varying currents in the outputs of the lowpass lters ,8, 13 are transmitted on a multiplex carrier basis by supplying to the modulators 9, 14 a plurality of waves of different frequencies obtained from multivibrator .16 which is supplied in turn with 60-cycle current from source 17. Band-pass filter 18 leading to modulator 9 in the pitch channel supplies 30cycle current to that modulator. vBand-pass filter 19 supplies 90-cycle current to the modulator 14 of the lowest-frequency channel. Band-pass filter 20 supplies S60-cycle current to the modulator 14 of the highest frequency analyzer channel. kThe filter 1li and the filters 15, respectively, pass the lower side-bands from the modulators 9, 14, the pass ranges being assumed to be 20 cycles in Width and lying between Ilimits given on the drawing in connection with each filter. There are thus transmitted overthe line L currents lying in the range l0 to 360 cycles. These are received in the apparatus R and separated by the filters 22 inthe pitch channel and 23 in the various spectrum channels.

For purposesof synchronizing the carrier some of the original 60-cycle current from source 17 may be sent through filter Z1 over the line L to the receiving point where it is picked off. by filter 24 and impressed on multivibrator 25 for generating a series of carrier frequency waves for the .various demodulators 27, 35. These frequencies are 'fed-to' the various demodulators through band-pass filters 256. For example, the demodulator 27 in the pitch control channel is supplied With 30-cycle current through band-pass filter 26 todemodulate the received pitch control currents which are passed through low-pass filter ZS'to control the switching. amplier 29 and to determine the fundamental frequency generated by relaxation oscillator 30.

The relaxation oscillatorl 30, noise source 31 and switching amplifier 29jmay be of the same type as dis-l closed in the Riesz vpatent referred to. When no voiced sounds are to be reproduced-for purposes of reconstructing'the speech, relaxation oscillator 3i) is biased vto zero outputy while switching amplifier 29 allows outputv from the noise source 31 to be sent into the supply circuit 32. When voiced sounds are to be produced, the ,switching amplifier 29 is biased beyond cut-off to prevent transmission of noise current into circuit 32 and the bias of relaxation'os'cil-lator 30v is shifted into its operative range tol cause currents simulating vocal cord vibrations of controlled fundamental pitch to be supplied to circuit 32.'

Spectrum ydefining currents transmitted through Various of the filters 23 are `demoduiated at 3S, these demodulators'being supplied with current of the proper frequency through filters` 26 as above noted. Corresponding slowly varying direct currents appear in the outputs of the several low-pass filters 36 leading to the modulators 37.

The Supply/circuit 32 is connected through various band-pass filters 38 to modulators 37 to supply current from the noisesource or relaxation oscillator, as the case may be, in different bands corresponding to the bands into which the speech was originally analyzed.

Patentedl Dec. 5, 1961 The outputs of the various modulators 37 are transmitted through band-pass filters 39 and equalizer 40 to the output sound source 41 which may be a loud-speaker, telephone line `or other utilization circuit. The equalizer 40 may be complementary to the equalizer 2.

So far as indicated up to this point, the type of transmission might be identical with that of the Dudley patent mentioned above in which, as pointed out, there is a considerable degree of privacy of transmission, since the currents transmitted over the line L are not understandable but require the use of specialized apparatus of the type shown at R for recovery of the speech message.

In accordance with the present invention, a high degree of priv-acy is obtained by transmitting the pitch control and spectrum control currents as reversed signals. The significance of this will be made apparent fromconsideration of the upper curve A of FIG. 7 which shows, with compressed time scale, the spectrum defining current for the 250 to S50-cycle channel taken from the oscillogram given in FIG. 14 of the Dudley article referred to. This current has zero -value in the absence of speech currents in 250 to S50-cycle band and the current rises to various amplitudes depending upon the variation of energy in this band with time. FIG. 7 assumes that the maximum current reached at any time is given by the horizontal line B. In accordance with the modification of the invention disclosed in FIG. l, the reverse of the current A is transmitted. That is, when no energy is present in the band, maximum current, corresponding to the value B, is transmitted and when the curve A rises to the maximum value B, zero current is transmitted. In other words, the difference current between curve A and line B is the current actually used for transmission.

The manner in which this is accomplished will be explained in connection with FIGS. 2, 3 and 4. FIG. 2 shows the arrangement of the modulator 14, band-pass filter 15, low-pass filter 13 and connection to modulator from 90-cycle supply filter 19. The modulator comprises push-pull vacuum tubes with the output of lter 13 connected across resistance 45 in the common grid lead in series with bias battery 46. The polarity and initial bias are such that modulator 14 gives maximum output with zero input and decreasing output with increasing input. This is indicated by the curve in FIG. 3, the operating point being shown at zero corresponding to large normal output. As the signal input increases in the direction of the arrow the modulator output decreases in substantially linear manner.

This same method of sending is used on the pitch control channel and each of the spectrum channels, so that in each case the transmitted current varies in amplitude in inverse manner to the strength of the modulating current. Such reversed currents would appear to an observer to have the same general form as the normal currentsv in thepitch control and spectrum control channels, but an attempt to reconstruct the speech message from such reversed signals would be unsuccessful. The silent and non-silent intervals are interchanged :and the`voiced' and unvoiced controls are interchanged. Also, the spectrum distribution of the speech vsounds would be completely changed. In frequency regions where the original speech sound had high energy content the synthesized speech would have low energy content, and viceV versa. The pitch changes would be the reverse of those existing in the original speech.

The transmitted waves are successfully received in the circuit R by using for each of the modulators 37 the same type of circuit -as shown in FIG. 2 but in this case adjusting the bias battery 46 and resistance 45 in phase and voltage value to give the type of operation disclosed in` FIG. 4. The normal operating point is at O in this figure, corresponding to minimum modulator output in response to maximum received current in each channel. As the current decreases from this normal operating value the modulator output increases.

In similar manner, switching amplifier 29 and relaxation oscillator 30 are each provided with such a bias that strong received current corresponding to unvoiced sounds makes amplifier Z9 conducting but relaxation oscillator 30 non-conducting. XVeaker received signal corresponding to voiced sounds shifts the' switching amplifier bias beyond cut-ott and shifts the grid voltage of relaxation oscillator 3'0 into its operating range. This can be accomplished by using suitable bias batteries and by applying the control current received over the pitch control channel to the switching amplifier and relaxation oscillator in proper polarity. Y

With the foregoing connections and adjustments of the switching amplifier 29, relaxation oscillator 30 and modulators 37, the transmission of the various subbands of frequencies through the modulators 37 of the synthesizer circuits is carried out inthe proper manner to reconstruct the speech in accordance with the operation outlined earlier in the description of the FIG. 1 system and described more fully in the Dudley patent.

Instead of sending the ysignals with reversed phase continuously, a greater degree of secrecy isV achieved by alternately transmitting normal and reversed signals. The circuit could be altered to accomplish this in any suitable manner, either under manual control or by automatic control, and the changes could be effected at relatively short or long time intervals as desired.

A system for altering the circuits at irregular intervals and in random manner under automatic control is disclosed in FIGS. 5 and 6 in which the general layout is the same as in FIG. 1 and the. same reference characters designate the same respective parts of the system. In the system of FIGS. 5 and 6, however, the reversal of phase of the signals at the transmitter and also at the receiver is effected in the channels carrying varying direct currents rather than in the modulating circuits.

Referring to FIG. 5, reversing relays50, 51 individual to the various channels are used to effect the reversal of phase. These relays are individually controlled by brushes 52 riding upon punched sheet 53, such as paper, driven by contact roller 54 on the `shaft of motor S5. Since there are eleven relays, one for the pitch control channel and one for each of the ten spectrum control channels, there are eleven brushes 52 registering with eleven rows of perforations, the latter varying in length and spacing in irregular, random manner, the punchings in one row being independent of those in the other rows. When one of the. brushes 52 comes to a perforation in 'sheet 52 it makes contact with metal cylinder 54 and Closes a circuit from grounded battery 56, brush S7, slip ring 58, shaft and roller 54, corresponding brush 52, and winding of the corresponding relay 50 or 51 to ground, causing suchrelay to attract its armatures.

The relay 50 is shown in energized condition. By

means of its two upper armatures the input circuit from frequency measuringV circuit 7 to low-pass filter 60 is reversed in phase when the armatures shift from upper to lower contacts. If thephase is considered normal when these armatures are in their upper position, then with the armatures attracted, as in the figure, the phase is reversed.

In accordance with the wave forms shown in FIG. 14 of the Dudley article referred to and shown in curve A, FIG. 7 of the present drawing, these various current waves lie above the zero current axis and it is desired that thereversed phase waves should also have the same polarity. In order to accomplish this the lower armature of relay S0, when attracted, connects battery 61 in series with resistance 62 and auxiliary low-pass filter 63 into the output circuit of filter 60 to add a fixed current to the reversedphase wave, this fixed current corresponding to the maximum value given by B in FIG. 7. Auxiliary filter 63 suppresses transients and rounds off the corners of the impulses resulting from the switching in and out of the battery 61.

A similar arrangement-is` used in4 each of the spectrum channels, relay 51 of the uppermost channel being indicated as in normal or unenergized condition at the time assumed.

Referring to FIG. 7,'thel time: of switching are indicated along the time axis at O, a, b m.y At time O there is a reversal of phase, so that in the interval O-a the portion of the signal wave A that is sent has the form shown by curve C in the corresponding interval. In the y interval a-b the current is not reversed but has the same form as the corresponding portionv of the wave A, except that the sharp pulse s (indicated by dotted lines) that would exist from the switching action of the relay is suppressed by the low-pass filter, giving a current of the form shown by the solid line. Similarly, at each successive switching point there is a reversal from normal to reversed pihase; or vice versa, and the corresponding tails of current s are suppressed kby the low-pass filter. As a result of this action thecu-rve C represents the character of the transmitted current, and it willbe seen that this l bears no recognizable relation to the signal curve A.

For purposes of synchronizing, the motor 55 maybe driven by current obtained from the source 17, for eX- ample current taken from the output of the filter 21.

The tape or perforated sheet 53 has, as stated, perforations vary-ing in length and spacing in s ome irregular or random manner. As an example of the actual switching times that might be employed for any one channel, these may lie between .O4 second and .22 second on the basis that .04 second represents the minimum syllabic interval and .22 second represents the maximum syllabic interval. The switching times indicated on FIG. 7 lie between theseV limits and are distributed along the time axis in a manner chosen at random. The minus signs indicate that the particular portions are reversed while the plus signs indicate no reversal.

The currents in the various analyzer channels and pitch control channel, after subdivision with alternate portions reversed as described, pass to the various modulators 9, 14 for modulation of the respective carrier waves for transmission. Wh-ile the band-pass filters 10, 15 are indicated as passing the same width of band as in the FIG. l system such band width may be increased as desired and the carrier frequency spacing also increased where necessary, if the various currents C require use of broader bands for satisfactory transmission.

Referring to FIG. 6, the received 60cycle current is used after suitable amplification to synchronize the motor 64 with the motor 55. Punched sheet 66 is a duplicate of sheet 53 so that brushes 67 make contact with roller 68 in identical time correspondence with the contacts between brushes 52 and roller 54. The relays 70, 71 are, therefore, individually energized and released in unison with relays 50, 51, with such allowance as may be necessary for time of transmission over long circuits.-

Relay 70 is shown with its armatures attracted, so that its upper two armatures effect a phase reversal as between demodulator 27 and low-pass iilter 72, resulting in restoration of phase of the pitch control wave. The lower armature connects battery 74 in circuit with resistance 75 and auxiliary low-pass filter 73 so as to elect a shift of the cu-rrent axis, along with the turning over of the phase, suicient to restore the portion O-a of current wave A, FIG. 7. Battery 74 should, therefore, be identical with battery 61, assuming zero transmission equivalent. Relay 71 of the uppermost spectrum channel is deenergized, so that the spectrum wave for this channel is at the instant under consideration being received in ynormal phase.

It will be clear tha-t the control of the various relays by the sending and receiving perforated sheets is such that the signals are either reversed or not as is necessary to restore them to their original form. The current Wave A, illustrating the current variations for one channel, is thus reconstructed from the transmitted current of the form C except for slight discrepancies due mainly to filter distortion as illustrated by the dotted lines drawn in on graph A.

. v 6 The restored pitch control currents areused to control the energy source of frequency rpatterns which may be of the same type as that disclosed in the Ries patent cited, *and which functions to supply to circuit 32 energy of continuous spectrum or energy simulating vocal cord Vibraf- Ations. Modulators 87 operate as in the Riesz or Dudley patents to reconstruct thev speech under control o f the re stored spectrum control currents in the various channels.

The invention is not to be construed as limited to the the energy variations in the' respectiveband, subtractingA each such slowly varying, current fro'mta constant value of current to produce inverse Yvariations,"and lsimultaneously transmitting said inverse variations. 'Y

2. The ymethod o-f secret signaling comprisinganalyzing speech waves .to derivetherefrom a plurality of slowly varying currents each indicative of the power content within a different narrow frequency band of the speech waves, each such current being in inverse proportion to such power content, and simultaneously transmitting said slowly varying currents to a distance.

3. The method of secret signaling comprising analyzing speech message waves to derive therefrom -a plurality of slowly varying currents varying at the order of Sylla-hicY frequencies or slower, one such current indicating funda-V mental pitch variations and the other of said currents indicating the power contents of a corresponding plurality of 4. The method ofk secret signaling comprising analyzing, speech message waves to derive therefrom a plurality of slowly varying currents varying at the order of syllabicfrequencies or slower, one such current indicating fundamental pitch variations and the other of said currents indicating .y the power contents of a corresponding plurality of frequency bands of the speech range, respectively, individuf ally producing reversals of said several currents from time to time independently of one another, and simultaneously transmitting said currents alternating with said reversals.

5. In telephony in which, in known' manner, the speech waves are analyzed into a pitch-defining signal and a plurality of speech-defining signals -before transmission and speech waves are synthesized from locally produced spectrum and noise energies under control of said pitch-defining and speech-deiining signals after reception of thelatter; the method of rendering the transmission secret comprising from time to time in irregular manner sending re versals of said signals alternating with normal signals and synchronously producing compensating reversals upon lreception, v

6. In speech privacy, means to analyze speech waves into a plurality of speech-defining signals of low frequency in separate circuits, means to derive the diierence current between each such signal and a current of xed value, means to transmit such diiference currents simultaneously, and means to reconstruct understandable speech therefrom.

7.-In speech privacy, means to analyze speech waves into a plurality of speech-defining signals, in which the amplitude of the signals is inversely proportional to the amount of power represented in respective narrow frequency bands of the speech, means to transmit such speech-defining signals, and means to reconstruct understandable speech therefrom. v

8. The method of receiving secretly transmitted speech in the form of a plurality of simultaneous spectrum control waves of low frequency, portions of which are of reversed phase, comprising reversing the portions. of said waves that are received-in reversed phase to restore said control Waves,y generating Waves simulating voiced and unvoiced sounds and selectively controlling portions of the. generated waves existing in different frequency bands in accordance with respective restored control waves to reconstruct understandable speech. y

9. In speech privacy, means to analyze speech waves into la plurality of speech-defining signals of low frequency in separate circutis, individual phase-inverting means in 10 said circuits, and means to render each said phase-inverting means alternately operative and inoperative during transmission to send out the corresponding speech-dening signal in. normal and inverted phase alternately.

10. A receiver for speech transmitted by the system 1 defined in claim 9 including means for inverting the phase of each of the, transmitted speech-defining signals, and means rendering each said last means alternately operative and yinoperative during reception in unison with the respective earlier-mentioned inverting means, to restore each inverted phase signal to normal phase, andV means to reconstruct speech message waves under control, of the received and restored speech-dening signals.,

l1. -In speech privacy,` means to -analyze speech waves into a plurality of speech-defining signals of relatively low frequencies in separate circuits, individual phase-inverting means in said circuits, intermittently operating switching mechanism for the respective inverting means, for rendering theindividual inverting means alternately operative and inoperative, inirregul-ar manner during transmission. t

Rferencs cited in Vthe me of this patent UNITED STATES PATENTs 2,098,956 Dudley L. NOV. 16, 1937 2,115,803 Dudley tMay 3, 1938 2,183,248 Riesz s Dec. 12, 1939 

